And scientists don’t know why. Perhaps we took over the species’ territory and hunting grounds. Perhaps we coupled with them, eventually mating them out of existence. Or perhaps, as has been suggested in other studies, we flat-out slaughtered them.

Why does this new study, published today in PNAS Online Early Edition, suggest that the interaction between the two human species was brutal and violent? It undercuts a long-held theory that Neanderthals and modern humans interacted for thousands of years. Instead, the researchers believe any co-existence between Neanderthals and modern humans is likely to have been much more restricted, perhaps to a few hundred years. And it was during this brief period of time that Neanderthals—a squat, rugged species that dominated Eurasia for the better part of 200,00 years— suddenly disappeared from the face of the earth. That’s pretty compelling circumstantial evidence.

The new study, published by scientists from University College Cork and the University of Oxford, uses an improved method of radiocarbon dating, based on a new way to exclude contaminants, to argue that most, and maybe all, Neanderthal bones in Europe are at least 39,000 years old. Until now bones from several Neanderthal sites have been dated to as young as 29,000 years ago, suggesting there was extensive overlap between the two human species as modern humans entered Europe around 44,000 years ago.

Richard Klein, a paleoanthropologist at Stanford University, told the New York Times that the study fit with his own view that “modern humans were technologically and intellectually far superior to the Neanderthals.” This, he told paper, “would have allowed them to spread very rapidly and to precipitate the extinction of the Neanderthals almost immediately on contact.”